1. Field of the Invention
The present invention relates to a liquid electrophotographic printer, and more particularly, to an apparatus for eliminating excess ions in a developer used for a liquid electrophotographic printer so that a decrease in the concentration of toner on an image can be prevented.
2. Description of the Related Art
In a general liquid electrophotographic printer, an image recording surface of a photoreceptor medium such as a photoreceptor belt or photoreceptor drum is electrically charged. Light is selectively scanned on the image recording surface so that the level of electrical potential varies and an electrostatic latent image is formed. Then, charged developer adheres to the electrostatic latent image to form an image.
FIG. 1 shows a general liquid electrophotographic printer. Referring to the drawing, the liquid electrophotographic printer includes a photoreceptor belt 14 which is operated by being supported by a driving roller 11, a transfer backup roller 12, and a steering roller 13, a laser scan unit 30 for emitting a laser beam onto the photoreceptor belt 14 according to an image signal to form an electrostatic latent image, and a developing unit 40 for developing the electrostatic latent image by adhering a developer to the electrostatic latent image.
In the case of a color printer, a plurality of laser scan units 30 for forming electrostatic latent images corresponding to colors, i.e., yellow (Y), magenta (M), cyan (C), and black (B) images, and a plurality of developing units 40 for developing the electrostatic latent image for each color are installed.
The electrostatic latent image of the previous step is removed as the photoreceptor belt 14 passes through an eraser 15. The photoreceptor belt 14 is charged to a predetermined uniform electrical potential at a corona unit 20. The laser scan unit 30 emits a laser beam and selectively lowers the electrical potential of the surface of the photoreceptor belt 14, i.e, the image recording surface, so that an electrostatic latent image is formed. Thus, a predetermined difference in electrical potential is generated between the electrostatic latent image and its surroundings on the photoreceptor belt 14.
The developing unit 40 develops the electrostatic latent image by attaching a developer including toner of a predetermined color to the image record surface of the photoreceptor belt 14 where the electrostatic latent image is formed. That is, when the developer including toner of a predetermined color and liquid carrier is supplied to the image recording surface of the photoreceptor belt 14, the developer selectively adheres to an area where the electrostatic latent image is formed due to the difference in electric potential.
As the photoreceptor belt 14 is operated, the liquid carrier of the developer attached to the position of the electrostatic latent image is absorbed and removed by a drying roller 71 and only toner particles are left at the position of the electrostatic latent image on the photoreceptor belt 14. The toner particles, i.e., a color image, is transferred to a transfer roller 73 installed to face the transfer backup roller 12. The transferred image is printed on a print medium 80, such as a sheet of paper, passing between the transfer roller 73 and the press roller 77. The image printed on the print medium 80 is fixed by an additional fixing unit (not shown).
As shown in FIG. 2, the developing unit 40 adopted in the above liquid electrophotographic printer is installed under the photoreceptor belt 14. The developing unit 40 includes a developing receptacle 42, in which a developing roller 41 and a squeeze roller 43 are installed close to the photoreceptor belt 14, and a mixing tank 45 in which a developer 47, which is a mixture of toner of a predetermined color and liquid carrier, is maintained at an appropriate concentration.
The developer 47 of appropriate concentration is contained in the mixing tank 45. The concentration of the developer 47 can be controlled by mixing a developer appropriately supplied from an ink vessel (not shown) where a developer of high concentration is contained and a carrier appropriately supplied from a carrier vessel (not shown) where a liquid carrier is contained. The developer 47 is pumped by a pump 48 toward the developing roller 41 through a developer supply pipe 49. Here, the toner of the developer 47 is charged to a predetermined electric potential.
The developing roller 41, as shown in FIG. 3, is rotated in the same direction as the photoreceptor belt 14, being separated by a developing gap (G) from the photoreceptor belt 14. According to rotation of the developing roller 41, the developer 47 supplied between the developing roller 41 and the photoreceptor belt 14 through the developer supply pipe 49 is attached to an electrostatic latent image area (A) of the photoreceptor belt 14. Here, since the electric potential of the electrostatic latent image area (A) is lower than the surroundings, as described above, the developer 47, particularly, toner having a predetermined electric potential, selectively adheres to the electrostatic latent image area (A). The squeeze roller 43 presses the developer 47 attached to the electrostatic latent position (A) and presses the toner close to the photoreceptor belt 14, while squeezing the liquid carrier.
The rest of the developer 47 which is not adhered to the electrostatic latent image position (A) of the photoreceptor belt 14 is collected in the developing receptacle 42. The collected developer is fed back to the mixing tank 45 and reused.
However, ions in the developer 47 increase in number as the developer repeats the reuse process. That is, the ratio (% FPC) of bulk conductivity of a developer including a liquid carrier and toner to free phase conductivity of a developer excluding the toner, i.e, the liquid carrier, increases. The excess ions, particularly cations, adhere to the electrostatic latent image area during development of the electrostatic latent image. Accordingly, the amount of toner adhering to the electrostatic latent image area decreases so that the intensity of the image decreases.